The present invention relates to a mesh electrode structure for an electron discharge device and more particularly to a planar mesh electrode structure that facilitates forming into a non-planar mesh electrode structure for a photomultiplier tube.
An electron multiplier is a device utilizing secondary electron emission to amplify or multiply the electron current from a primary electron source, such as a photocathode or a thermionic cathode. The usual electron multiplier comprises a series or chain of secondary emitting elements, called dynodes, interposed between a primary electron source and an output collector or anode. The electrodes are constructed and arranged to form an electron optical system for directing primary electrons from the primary source onto the first dynode releasing therefrom several secondary electrons or "secondaries" for each primary electron. These secondaries are directed by the electron optical system onto the next dynode where each produces more secondaries. This process is repeated at each succeeding dynode or "stage" of the multiplier, thus producing a greatly multiplied electronic current from the final dynode to the collector. The number of dynodes or stages may be from one to twenty or more depending on the amount of amplification needed. Each succeeding dynode in the chain is maintained at a potential substantially higher, e.g., 100 volts, than the preceding dynode, to accelerate the secondaries from dynode to dynode. The dynodes are preferably shaped to direct and focus the electrons emitted therefrom to the next dynode.
Electron multipliers are particularly useful for amplifying electron currents produced by weak signals, such as light or nuclear radiation. When used for detecting and/or counting rapidly recurrent signals such as nuclear particles, it is necessary that the multiplier have sufficient speed and a resolving time low enough to distinguish between successive signals or particles.
The speed of a multiplier can be increased by reducing the overall transit time of primary and secondary electrons between the primary electron source and the collector. The resolving time of a multiplier is limited by the transit time spread of electrons through the multiplier chain, that is, the difference between the transit times of the fastest and slowest electrons. This transit time spread is primarily due to differences in the trajectories taken by various electrons through the multiplier and differences in the initial velocities of secondary electrons. A structure for improving the electron transit time through the multiplier is disclosed in copending U.S. Pat. No. 4,431,943 filed on Oct. 14, 1981 by Faulkner et al., assigned to the same assignee of the present application and incorporated herein for disclosure purposes.
In photomultiplier tubes, the speed or transit time of the tube is a function of both the photocathode transit time difference and the transit time of the electron multiplier. The photocathode transit time difference, defined as the time difference between peak current outputs for simultaneous small-spot illumination of different parts of the photocathode, is longer for edge illumination than for center illumination because of the longer edge trajectories for photoelectrons and the weaker electric field near the edge of the photocathode. In a planar photocathode, the center-to-edge transit time difference may be as much as 10 nanoseconds; whereas for spherical-section photocathodes, such as that shown in FIG. 1, the transit time response is more uniform because the electron paths are nearly equal in length.
The photocathode transit time difference is ultimately limited by the initial velocity distribution and angular distribution of the photoelectrons. These distributions cause time broadening of the electron packet during its flight from the photocathode to the first dynode. The broadening effect can be minimized by increasing the strength of the electric field at the surface of the photocathode. One way of increasing the electric field at the surface of the photocathode is to locate a mesh electrode a small distance from the cathode; however, in tubes having spherical-section photocathodes, it is difficult to form a spherical-section mesh. One method of forming such a mesh is described in U.S. Pat. No. 4,060,747 issued to R. D. Faulkner on Nov. 29, 1977 and incorporated by reference herein for the purpose of disclosure. The Faulkner patent discloses a domed mesh electrode having nonuniform apertures formed by stretching a planar metal member to achieve the non-planar shape. Frequently, the mesh wires break during stretching and the torn mesh must be disgarded. Thus, it is desirable to be able to form a non-planar mesh electrode while eliminating or minimizing the stretching experienced by the mesh wires.